An increase in sensitivity to the pressor effects of tyramine, and indirectly acting sympathomimetic agent, is a well known side effect of MAO inhibitor treatment. Using an intravenous tyramine steady state infusion technique we have studied the effects of the selective inhibition of the A and B forms of MAO on tyramine's pressor effects in patients treated with clorgyline, pargyline and deprenyl. After four weeks of treatment, clorgyline produced a significantly greater increase in tyramine sensitivity in comparison to a medication-free baseline (29-fold) than did pargyline (12-fold) or deprenyl (1.7-fold). This data suggests that the tyramine sensitivity which develops during MAO inhibitor administration is principally due to a reduction in MAO type A activity. A high correlation observed between changes in tyramine sensitivity and reduced plasma 3-methoxy,4-hydroxy phenylglycol concentrations supports this interpretation. Clorgyline administration was also found to be associated with reductions in tyramine-stimulated plasma norepinephrine release.